Yes, Vivo Y91 does have the memory card slot. You can use 2 Sim cards and a MicroSD card at the same time.
No. Vivo Y91 does not have the front camera flash but it does have the display flash. Which helps to capture good selfies in low light conditions.
No. Vivo Y91 is not a waterproof mobile phone.
Yes, Vivo Y91 does have the VoLTE support.